It is generally believed that a CMOS, very large scale integrated (VLSI) circuit, is highly sensitive to .gamma.-radiation and that circuit failure due to exposure to .gamma.-radiation is caused by a shift in the threshold voltage of the field effect transistors (FETs) in the circuit.
The proposed mechanism on which such a belief is based differs for N-channel and P-channel field effect transistors (NFETs and PFETs) as follows. When .gamma.-radiation (photons) penetrates the thin gate oxide of an NFET, electron-hole pairs are created in the oxide. The electrons are quickly swept away from the oxide. The holes move more slowly. Many are trapped at the Si-SiO.sub.2 interface, thereby creating positively charged traps. At relatively low .gamma.-dosage, the extra positive charge is operative to reduce the threshold voltage of the NFET, an effect which is enhanced when a positive gate voltage is applied to the NFET. At very high .gamma.-dosage (in excess of 10.sup.6 rad [absorption dosage in silicon]), migrating holes appear to create negatively charged traps at the interface by some mechanism which is, as yet, not understood in the art. The negatively charged traps, and the threshold voltage as well, increase with increasing dosage.
In PFETs, the effect due to .gamma.-radiation is different. In a PFET having a negative gate voltage applied, the fast moving electrons of the electron-hole pairs created in the gate oxide are swept away mostly to the channel and the slow moving holes are collected by the gate electrode. Holes are not trapped to any significant extent within the oxide. Therefore, the effect of .gamma.-radiation on the PFET threshold voltage is much less than that of the NFET.
The above mechanism leads one to the conclusion that the threshold voltage shift, for either NFETs or PFETs, is (amongst other things) proportional to both the number of electron-hole pairs created and the number of possible trapping sites. Since both the number of trapping sites and the number of electron-hole pairs are proportional to the oxide thickness, the threshold voltage (shift) is proportional to the square of the thickness of the gate oxide.
The above-described proposed mechanism has led some workers in the art to the optimistic projection that radiation effects would be small in very small feature size technology with very small gate oxide thicknesses below 250 Angstroms. This projection was seemingly confirmed in testing of random access memory (RAM) and some microprocessors reported in the literature (see, for example, R. T. Davis, M. H. Woods, W. E. Will and P. R. Measel, "High-performance MOS resists radiation", Electronics, Nov. 17, 1982, pp. 137-139). The optimism is based on an extrapolation of the functional dependence of the threshold variation on the gate oxide thickness into the range of such very small oxide thicknesses. Applicant's work indicates that such an extrapolation is unwarranted and that undesirable threshold variation effects will continue to be a problem even in the case of very thin gate oxides.
Moreover, threshold shifts due to other causes also produce failres in CMOS circuits. It would therefore be desirable to have CMOS circuits which are less susceptible to failures due to threshold shifts.